The specific aim is to determine the hormones regulating the growth and regression of follicles and corpora lutea in the hamster and rat. REPRESENTATIVE PROJECTS INCLUDE: 1. Preliminary experiments indicate that low doses of exogenous progesterone can block estrogen secretion by antral follicles without altering serum levels of LH, FSH or prolactin. This problem will be extensively studied in vivo and in vitro models to determine the mechanisms. 2. What morphological, hormonal and biochemical changes occur in antral follicles (40-50 per hamster) undergoing atresia at the same zero time? 3. Is estrogen a direct luteotropic hormone in the pregnant rat in the second half of gestation? 4. What hormonal changes are elicited by injection of anti-LH serum in the pregnant hamster and rat. 5. Ovulation can be induced in hypophysectomized, proestrous hamsters by FSH or LH but progesterone is only secreted by the CL induced by LH. What factors are involved in this difference from the CL induced by FSH?